1. Field of the Invention
This invention relates to heat-resisting cast steels which can be used as structural materials for the manufacture of pressure vessels such as the casings of steam turbines for thermal electric power generation.
2. Description of the Related Art
Conventionally, used high-temperature casing materials used in steam turbine plants for thermal electric power generation include 2.25% CrMo cast steel, CrMo cast steel, CrMoV cast steel and 12 Cr cast steel. Among these cast steels, the use of cast steels comprising low-alloy steels such as 2.25% CrMo cast steel, CrMo cast steel and CrMoV cast steel is restricted to plants having a maximum steam temperature of 566.degree. C. because of their limited high-temperature strength. On the other hand, 12 Cr cast steel (e.g., those disclosed in Japanese Patent Application No. 59-216322 and the like) have superior high-temperature strength to cast steels comprising low-alloy steels, and hence can be used in plants having a steam temperature up to approximately 600.degree. C. However, if the steam temperature exceeds 600.degree. C., 12 Cr cast steel has insufficient high-temperature strength and as such, are rarely used for pressure vessels such as steam turbine casings.